Wear my F5 also 24/7 with the original black silicone band...no problems til now.
I can imagine it could be a problem with sweating - i would consult a dermatologist.
I have just had exactly the same issue. Just got a new Fenix 5 plus titanium with solar orange strap and worn it for the first time whilst skiing for the last 10 days. A rash occurred on my left wrist so I moved it to the right wrist and that caused a rash too. Never had any issues with the Fenix 3 stars at all for the 2 years I had it. Strap composition is clearly not right.
3 years Fenix 5x, original black, forest green, and yellow bands. 4x50 minute runs every week and 20+ 4-5 hour rides each year. Never had a band or skin irritation. Always rinse device and towel dry post-activity. Never wear in shower. Otherwise, worn 24/7. No issues with titanium band either.
I use a titanium band for 50$ from china.
Well this is embarrassing. I just developed a rash on my left wrist after starting a new rowing machine workout at the gym. Before that it was mostly weight training. The rowing workout causes me to sweat and from what I can tell, my Fenix 5 does not like it when you sweat. Guess I need to start a post-workout band cleaning habit until I can get something better than the standard silicone band it came with. I'll try to sweat less too.